Cardiac myocyte accumulation of calcium in burn injury: cause or effect of myocardial contractile dysfunction.

Abstract

Myocardial calcium accumulation and myocardial injury occur after burn trauma. However, whether altered calcium dyshomeostasis occurs as a result of myocardial injury/dysfunction or whether altered calcium handling initiates myocardial injury and contractile abnormalities remains unclear. In addition, the specific mechanisms by which burn injury promotes calcium entry into cardiac myocytes, specifically L-type channels and the sodium-calcium exchanger, remain unclear. This study addressed the hypothesis that burn trauma promotes cardiomyocyte calcium accumulation, in part, via reverse mode function of the sodium/calcium exchanger and via L-type channels. Myocardial calcium accumulation, in turn, alters performance. Burn trauma (40% TBSA and sham burn for controls) was accomplished in Sprague-Dawley rats. Burns received fluid resuscitation (lactated Ringer's at 4 ml/kg/% burn). Hearts were harvested at several time points after burn injury (2, 4, 8, 12, 24, 48, 72 hours, and 8 days after burn) and were perfused with collagenase/bovine serum albumin-containing buffer to produce enzymatic digestion. Myocytes were then resuspended in MEM buffer, loaded with 2 mug/ml Fura 2AM for 45 minutes or 2 microg of sodium-binding benzofurzan isophthalate for 2 hours at room temperature in the dark. Cells were washed to remove extracellular dye and placed on a glass slide on the stage of a Nikon inverted microscope interfaced with Grooney optics. A computer-controlled filter changer allowed alternation between 340/380 excitation wavelengths; fluorescence was measured at 510 nm. Cardiac function (Langendorff) was measured in parallel groups at each time period (n = 6-7 hearts/time point). Cardiomyocyte accumulation of sodium occurred before alterations in myocyte calcium levels, and sodium/calcium dyshomeostasis preceded cardiac contraction deficits. Interventions that altered calcium flux through L-type channels (amlodipine) or sodium/calcium exchange (amiloride) attenuated burn-related myocyte calcium accumulation and improved contractile function. Our finding that myocyte sodium loading precedes myocyte calcium accumulation suggests a role for the reverse mode function of the sodium/calcium exchanger in burn trauma.